Heat oil in a large Dutch oven and add onion, celery, carrots and garlic. Season with salt and pepper. Cook until vegetables have softened, about 10 minutes. Add tomato paste and stir to coat vegetables.

Add chicken stock and lentils and bring to a boil. Reduce heat to low, cover and continue cooking about 30 minutes, stirring occasionally, until lentils are fully cooked. At this point you can puree half the soup to make it thicker, or not. Add the shredded rotisserie chicken and heat through. Serve with grated Parmesan cheese.